The present invention is related to particleboard furnish blending apparatus and more particularly to such apparatus using airless atomizing and application of resin and other liquid additives to a free falling furnish.
Conservation of composition board binder, whether it is the synthetic resin used presently or proposed resins based on wood lignins, bark extractives or other such binders, is extremely important to the wood industry and will be more important in the future. Present commercial blenders are generally considered to be inefficient. Consequently, the development of efficient blenders is a logical and high priority research need.
Particleboard and dry process fiberboard are produced from small particles or fiber bonded together by synthetic adhesives. Urea formaldehyde resin is most frequently used in production of these materials because of its lower cost in comparison with phenolic resin, both in terms of unit cost and speed of curing in a hot press. Phenol formaldehyde binder is favored for structural applications, particularly where durability is concerned. Binder cost in particleboard production can represent approximately 30% of the cost of the finished particleboard panel. Significant cost savings can therefore be realized with only slight improvement in binder efficiency.
In general, a uniform distribution of very fine droplets of resin over all wood particles in a furnish is most desirable for developing optimum board properties. Investigations have shown that application of a good distribution of small droplets of resin over the furnish is apparently the most efficient way to use binder. This, however, has been difficult to accomplish in industrial size blenders.
Short-retention-time blenders are favored (as opposed to long-retention-time blenders) for most plant use at the present time because of their small size, ease of maintenance, the ability to chill their shells more easily to reduce resin buildup, elimination of the need for compressed air at spray nozzles, and the reduction of possible air pollution developed from resin overspray. These types of blenders can damage fragile flake furnishes, thus long-retention-time blenders are favored with such furnishes.
Relatively new short-retention-time blending systems using "centrifugal" applicators have provided benefits to plant production. Such benefits as lowered maintenance costs, reduction of resin buildup within the blender and fewer resin spots on finished boards have resulted from use of centrifugal blenders. However, in general, the resin efficiency has not been improved.
Centrifugal blenders typically make use of rotating radial resin dispensing arms for airless "atomization" of the resins. Laboratory experimentation with many different forms of such apparatus has shown, however, that atomization of the resin does not occur in all cases. Instead thin streams of resin are often applied to the particles. Since all particles cannot be equally exposed to the resin streams, resin distribution must be accomplished as the particles rub together. It is not desirable to rely on rubbing action for resin distribution because many individual particles, particularly flakes, can become damaged in the process. The end result can be a low quality board.
Another problem with centrifugal applicators, as well as other forms of blenders, is that the furnish may not move through the blender in a continuous, unbroken "shell". Rather, a helical path is often developed, exposing inner walls of the blending housing. This means that part of the furnish is not exposed to the resin. Instead the resin meant for particles is applied to the walls of the housing, causing a decrease in resin efficiency and a difficult clean-up problem. An attempted solution is to cool the blender housing to reduce resin build-up. A film of condensate is formed on the inside of the blender and on the rotating blending shaft.
Airless atomizing experiments and industrial use have indicated that it is desirable to remove air turbulence from within blenders. Such removal substantially decreases the likelihood of escape of pollutants into the surrounding environment.
Airless atomizing has been previously accomplished through the use of pressure nozzles and high speed rotating disks upon which the liquid resin is applied. Pressure nozzles are subject to severe wear problems. In disk atomizing, resin received on the disk will flow immediately to the peripheral edge of the disk and be thrown radially therefrom in relatively small droplets. Such atomizers typically make use of a plurality of flat circular disks. The resin is delivered either through a central rotating shaft that mounts the disks or by an auxiliary delivery system. Care is required in the application of the resin to the disk surfaces in order that uniform atomization will result. Resin improperly deposited on a disk runs to its edge and is thrown in a random fashion. Small rivulets of resin tend to form on the disk, with the result that resin is discharged in relatively large droplets in a random pattern. Furthermore, the resulting "spray pattern" is very narrow axially, due to the perpendicular relationship of the disk surface relative to its rotating axis. The droplets tend to leave the disk in the same plane as the disk surface.
Another problem associated with short retention time blenders is that a uniform concentric shell of furnish is not typically formed within the blender housing. Therefore, it is not unusual that resin will be applied to the walls of the housing rather than the furnish. Build-up of resin on the housing walls can obviously result in waste and maintenance problems unless it is arranged to have the blender heavily loaded with furnish in order that the resin is wiped from the blender walls by the furnish. It is therefore desirable to produce and maintain a continuous concentric shell of particles passing through the blender housing to reduce the amount of resin striking the blender wall.
The present invention represents a substantial improvement over existing industrial forms of particleborad furnish blenders. It produces a uniform, airless atomized resin which is applied to a uniform shell of furnish falling axially within a concentric housing. Resin efficiency is increased and release of pollutants is minimized.